Sidescan sonar was originally developed by Dr. Harold Edgerton at the Massachusetts Institute of Technology in the 1960's. Current side scan sonar systems are towed from a platform, such as a ship or helicopter, hull-mounted on a surface ship, or carried on an undersea vehicle. The undersea vehicle can be an autonomous undersea vehicle (AUV) or a towed undersea vehicle 10, as shown in FIG. 1. The sidescan sonar system transmits an acoustical beam on each side of a transducer, or “fish”. The beams are sent in a wide angular pattern down to the bottom in swaths 50-500 meters wide, and the echoes are returned creating a narrow strip below and to the sides of the transducer track.
The undersea vehicle is often equipped with a pressure or altimeter sensor that allows it to follow the bottom while maintaining a constant height above the seafloor or alternatively “fly” at a constant depth below the surface. Important measurements such as heading, pitch, and roll are recorded on-board the AUV or, in the case of towed transducers, are often transmitted up the towing cable and recorded separately on the towing vessel.
Surveys are frequently accomplished as a series of parallel tracks over the seabed, with some overlapping area to ensure that the nadir of one survey track is covered by another survey track. Survey planning is discussed generally in “Hydrographic Work Flow—From Planning to Products”, by D. Cronin et al., 2003.
Because the Global Positioning Systems (GPS) does not function underwater, the position of cable-towed transducers is calculated from the GPS location of the surface vessel by using a cable layback model or acoustic tracking system.
FIG. 1 depicts a side scan sonar on an undersea vehicle being towed over the seafloor. The sonar beams strike the seafloor and are reflected back to the sonar system. Processing and sampling the raw sidescan data forms scanlines that make up grayscale side scan imagery (SSI), shown in FIG. 2. The side scan sonar cannot image the area of the seabed directly below the fish due to the spreading of the beams. This area is known as the nadir.
Objects close to or on the seafloor, such as mines, can be detected with SSI. These objects, or contacts, show up in the SSI as bright spots with adjacent shadows that face perpendicular away from nadir. Features of various shapes and sizes can be detected by the shadows, and the size of the shadow varies as a function of beam angle and feature dimensions. FIG. 3 shows an example of a small image extracted from SSI called a snippet that contains a contact. The U.S. Navy's Naval Oceanographic Office (NAVOCEANO) maintains a Master Contact Database (MCDB) containing thousands of features detected from SSI worldwide since the early 1990's.
The SSI is stored in NAVOCEANO's Unified Sonar Image Processing System (UNISIPS) format. Each scan line of the imagery is stored in a separate record, and the latitude and longitude coordinate, the sonar heading, speed, and depth above the seafloor are given for the sample at nadir.
By its nature, SSI is non-linear. Further, correlation between pixels in SSI is affected by “speckle” noise, which can also hamper the extraction of targets and features in sidescan imagery. In addition, the side scan imagery can contain phantom features created by surface return when the sonar is in shallow water or when the fish is pitched up or down. GPS signal dropout at the surface can also cause the imagery processing software to lose scan lines or to duplicate others.
Bottom objects and features can also change and migrate over time due to ocean currents and burial by sediment. Even when the objects are stationary, one of the biggest issues with sidescan sonar imagery is position error, often observed to be 15 m or greater during actual surveys. The center latitude/longitude position of each scan line in the UNISIPS file includes an estimate of position error due to GPS error and error from the cable layback model.
Change detection is the process of identifying differences in the state of an object or phenomenon by observing it at different times. The term “digital change detection” applies when computer algorithms are used to perform related change detection tasks commonly on data collected by a remote sensing device. Digital change detection is a major application of remote-sensed data, and some change detection techniques have been applied to satellite imagery and sonar imagery.
Currently, a side scan sonar survey is accomplished, and operators display and examine the side scan sonar data after the survey is complete. If the operator sees something that may be a contact on the side scan sonar image, the operator looks through the MCDB to see whether there is a nearby contact, and pulls up the UNISIPS format data and image from one or more previous surveys. The operator then compares the “new” side scan sonar image with the images of contacts in the nearby area and determines whether the new contact is the same as a previously found contact.